The assignee of the present invention, Volk Enterprises Inc., manufactures and sells a variety of pop-up food grade temperature indicators that typically are pre-inserted into a food product, such as whole chickens or turkeys, and sold to consumers with the poultry or meat product and are designed to be cooked with the product. During cooking, when the inner tip of the temperature indicator reaches a specified temperature particular to a metal alloy within the temperature indicator, the alloy melts, allowing an interior spring to force a stem to "pop up," indicating that the food product has reached the desired temperature. While these pop-up temperature indicators are widely used, there is a need to provide an improved temperature indicator design that achieves a total seal for the components of the temperature indicator.
U.S. Pat. No. 3,090,236 of Nicol, entitled "Temperature Indicating Devices," discloses a temperature indicator with a totally sealed temperature indicator enclosure in which are held two different aqueous solutions separated by a non-aqueous solid layer and a ball bearing. When inserted into, for example, a frozen food product, the solid, non-aqueous layer melts when the food product has thawed, and the two aqueous solutions mix and produce a substantially permanent coloration dissimilar from the color of the liquids prior to mixing. The solid, non-aqueous layer is designed to melt at temperatures in the range of thawing temperatures for common food items, with the patent giving specific examples for temperatures ranging from -20.degree. F. to 79.degree. F. While the temperature indicator of Nicol appears to provide a suitable indication for determining when food products have thawed, its design and application are limited to a narrow range of relatively low temperatures, due to the nature of the aqueous solutions and the plasticised PVC or polyethylene tubing used for the indicator device.